The present invention relates to a process and apparatus for molding thin walled containers with comparatively thick lips such as heat sealable food containers, vending cups and beverage tumblers.
Molded articles such as thin walled containers having comparatively thick lips or rims have traditionally been produced by injection molding, usually with a hot runner mold directly gated into the bottom of the container. Limitations on molding cycle and minimizing the sidewall thickness are imposed by the thickness and configuration of the article's lip. In some container styles the lip is considerably thicker than the side wall, typically a lip of 0.030-0.040" compared to a sidewall thickness of 0.015-0.020". It becomes increasingly difficult to properly pack the lip and to eliminate sink mars, when the resin must be forced through the restriction of a very thin sidewall from the gate at the bottom of the part. Generally, the resin must be raised higher in temperature to promote flow through this thin section. The consequences of doing this are that the molding cycle is slowed by the time taken to cool the thickest part of the container - the lip.
Injection blow molding is one approach which successfully overcomes most of the problems with a thick lip/thin sidewall configuration. U.S. Pat. No. 4,540,543 to Thomas shows a typical injection blow system for molding thin wall containers. However, this method requires specialized molds and expensive support equipment.
One problem which does remain however is the tendency to create a mold separation force when injecting the resin into the bottom of the part. This occurs because the top of the mold core acts as a post. As the resin flows under pressure against the top of the mold core, the mold core is pushed away from the mold cavity. The mold separation force thus created can be quite large because the relative cross section mold area at the bottom of the part is relatively large.
Another method for forming molded articles having comparatively thick lip portions is shown in U.S. Pat. No. 4,622,002 to Bormuth. A split cavity mold is shown with an edge gating system feeding directly into the lip of the part. The primary disadvantage of this arrangement is that the alignment of the cavity halves with the core will not adequately resist the considerable side load induced by the injection of the resin at the lip. This side force will cause the mold core to shift out of axial alignment with the mold cavity before the resin has filled the cavity. This in turn will result in a part having an unequal sidewall thickness. A second disadvantage is that the mold must contain complicated mechanisms to synchronize the opening and closing of the cavity halves during the cycle and the runner system must be arranged so the runner axis is perpendicular to the core, and the axis of the machine. This entails additional complexity and expense. A third disadvantage is the formation of gate vestiges on the outer perimeter of the lip.
Accordingly, it is an object of the present invention to provide a process and apparatus for injection molding thin-walled plastic articles with a comparatively thick lip portion.
It is a further object of the present invention to provide a process and apparatus as above which accomplishes direct lip gating without sacrificing core/cavity alignment and conventional mold motions along the machine axis.
It is yet another object of the present invention to provide a process and apparatus as above which forms a thin-walled plastic container having a comparatively thick lip portion with substantially no gate vestiges on the lip sealing surface or on the outer perimeter of the
These and other objects and advantages will become more apparent from the following description wherein like reference numerals depict like elements.